Remote radio heads (RRHs) are becoming heavily deployed in modern communication systems to function as front-end devices for performing digital antenna processing as well as analogue radio frequency (RF) functions. The main use of RRHs is to serve as subsystems of a distributed antenna system (DAS), such as illustrated in FIG. 1. Compared to a centrally located base station antenna system, a DAS offers the advantage of a uniformly high capacity due to the higher density of RRHs in comparison to a centralized macro cell base station, and substantially eliminates coverage holes due to the shorter distance from the average user to the closest RRH.
FIG. 2 illustrates a simplified block diagram of a MIMO (multiple input multiple output) DAS 200. The base station controller 210 performs bit-level processing including channel coding, scrambling and interleaving, followed by modulation of data to be transmitted to produce scrambled and modulated codewords. Layer mapping is then performed on the scrambled and modulated codewords to create a multitude of parallel data streams intended for transmission to one or more users. Subsets of the parallel data streams are then transmitted (via wired or point-to-point wireless interfaces) to multiple RRHs 220. Each RRH 220 is capable of performing digital processing on the received subset of data streams followed by digital to analogue conversion and analogue processing before the subset of data streams are finally transmitted via an antenna subsystem. Accordingly, within such a MIMO DAS 200, multiple RRHs may be transmitting data streams substantially simultaneously, resulting in a high potential for inter-RRH interference.
In order to avoid inter-RRH interference, it is known to perform precoding of the data streams prior to the data streams being transmitted to the RRHs 220. Such precoding enables the signals transmitted by the RRHs to include interference cancellation components that cancel out inter-RRH interference at the receiving user equipment. Conventionally, such precoding is performed centrally at the base station controller 210. However, such centralised precoding requires channel feedback relating to each RRH antenna from all RRHs to be sent to the base station controller via the wired or point-to-point wireless interfaces, which requires a significant communication channel overhead between the base station controller and each RRH. This communication channel overhead becomes prohibitive in massive MIMO architectures, which can consist of up to 256 antennas.